Propagate errors in places that are already set up to handle them, like
WebGLRenderingContext and the Tubes demo, and convert other callers
to using MUST.
glCullFace only accepts GL_FRONT, GL_BACK and GL_FRONT_AND_BACK.
We checked if the mode was valid by performing
```
cull_mode < GL_FRONT || cull_mode > GL_FRONT_AND_BACK
```
However, this range also contains GL_LEFT and GL_RIGHT, which we would
accept when we should return a GL_INVALID_ENUM error.
glDeleteTextures previously did not check that the texture name was
allocated by glGenTextures before adding it to the free texture name
list.
This means that if you delete a texture twice in a row, the name will
appear twice in the free texture list, making glGenTextures return the
same texture name twice in a row.
